1. Field of the Invention
The present invention relates generally to integrated circuits, and more particularly, but not exclusively, to metal oxide semiconductor transistors.
2. Description of the Background Art
A metal oxide semiconductor (MOS) transistor comprises a source, a drain, and a gate. The source and the drain are typically formed in a substrate, such as a silicon substrate. The gate is formed over a channel region separating the source and the drain. By controlling a voltage on the gate, current flow through the channel region may be modulated. In a depletion-type MOS transistor, a control voltage may be applied on the gate to pinch-off the channel and prevent current flow between the source and the drain. In an enhancement-type MOS transistor, current flows between the source and the drain when a channel is formed in the channel region as a result of a control voltage on the gate overcoming a threshold voltage.
A MOS transistor is referred to as being “ON” when current flows through it, and “OFF” when there is no current flow. The speed at which a MOS transistor can be switched ON and OFF is referred to as “switching speed”. Regardless of its type, a MOS transistor preferably has relatively fast switching speed. One way of improving switching speed is by lowering interface contact resistance between layers of the gate electrode.